The present invention relates to a hanger system for cementitious building units. In particular the present invention relates to an adjustable hanger system for precast cementitious building units.
The present invention is based on the provisional specification filed in relation to New Zealand Patent Application No. 571533 the entire contents of which are incorporated herein.
Precast concrete flooring units have been in use for a number of years and have become particularly prevalent in the construction of multi-story buildings.
These precast concrete flooring units typically are in the form of a “double tee” whereby bottom surfaces of the overhanging tee in the form of a web are supported on building support beams. More recently, in an effort to improve the performance of the flooring unit, the webs have been replaced with “dapped ends” (halved and reinforced ends) or flange supports (where a flexural tension reinforcement is added to the web) in an effort to reduce serviceability issues relating to the movement of the supports after installation by reducing the depth of the tee and bringing the support level closer to the centre of gravity.
Referring to FIG. 1, traditionally, hangers for double tee or rib-and-infill concrete flooring units have included a steel rebar (a) cast into a concrete slab (b), with an exposed section of the rebar welded to a heavy steel billet or box section (c) to provide a cantilever overhang for attachment of the concrete flooring unit to a structural beam or column. This method of construction is referred to as the “Cazaly method”.
Disadvantages of this type of hanger are that they are heavy, making installation of the concrete flooring unit difficult. The unit itself is also expensive to manufacture, and requires welding of the rebar to the cantilever billet.
Referring to FIG. 2, attempts to overcome these disadvantages include the use of a more lightweight loop-bar or pigtail (d) which has its elongate ‘leg’ (e) cast into the concrete slab (f) and the loop (g) exposed to provide anchorage for a concrete top overhang portion (h) that supports the concrete flooring unit.
Disadvantages of this system include:                lack of durability and relative lack of resistance to frame elongation and rotation of the support beams as a result of seismic activity; or        shortening of the precast units due to creep, shrinkage, temperature change and/or repeated loading. Under estimation of the performance of the pigtail system can have severe safety consequences to the stability of the resulting building structure. The pigtail system was reviewed by the Structural Engineering Society of New Zealand (SESOC) who issued a warning document on 12 Jan. 2009 recommending that pigtail hangers are not used in New Zealand and at present they are not considered compliant for hanging double tee concrete floors. In addition, hollow core building elements are usually placed on top of a building support surface rather than hung below the support surface which can result in a relatively unstable structure if the support surface is moved (e.g. as a result of seismic activity) due to the higher centre of gravity.        
A further significant disadvantage of both the rebar and pigtail concrete hangers is their lack of adjustment for use with flooring units of different thicknesses. This necessitates a manufacturer of such concrete building units to stock different sized hangers which can result in increased costs. In addition, the “pigtail hanger” is relatively expensive to manufacture.
It is an object of the present invention to provide a hanger system for concrete building units which address the foregoing problems or at least to provide the public with a useful choice.
It is acknowledged that the term ‘comprising’ may, under varying jurisdictions, be attributed with either an exclusive or an inclusive meaning. For the purpose of this specification, and unless otherwise noted, the term ‘comprising’ shall have an inclusive meaning—i.e. that it will be taken to mean an inclusion of not only the listed components it directly references, but also other non-specified components or units. This rationale will also be used when the term ‘comprised’ or ‘comprising’ is used in relation to one or more steps in a method or process.